1. Field of the Invention
This present invention relates to a phase lock loop, and more particularly, the present invention relates to a phase lock loop applied in wireless communication systems.
2. Description of the Prior Art
In a wireless communication system, a phase lock loop is used to generate a transmission signal that has specific phase and specific frequency. In a conventional phase lock loop, the specific frequency is generated through the following steps: first, generating a local oscillating frequency by a frequency synthesizer; next, dividing the local oscillating frequency by a fixed divider and inputting the signal with a reduced frequency into the phase detector; then, generating a control signal; and finally, generating the specific frequency by the control signal. Therefore, for the same specific frequency, the corresponding local oscillating frequency is fixed. However, because of the noise interference, the signal quality of the corresponding fixed local oscillating frequency becomes worse, and the signal quality of the transmission signal of the phase lock loop is also worsened.
There are several kinds of frequency synthesizer, but each of them has different kinds of noise interferences. For example, the interference caused by the multiplied frequency of the crystal oscillator frequency, that is when the desired local oscillating frequency approaches the multiples of a certain spur frequency, the local oscillating frequency is easily interfered. Thus, around the desired local oscillating frequency from several kHz to 1 MHz, there will be some interference signals.
The conventional method for preventing the noise interference focused on the improvement of the frequency synthesizer, but that will make the circuit more complicated and increase the cost of production. When the problem of interference remains unsolved, that frequency might not be usable, or that frequency might have a lower usage quality.